Long time constant servo system



United States Patent LONG TIME CONSTANT SERVO SYSTEM Francis P. Coflin,Jr., Schenectady, N. Y., assignor, by mesne assignments, to the UnitedStates of America as represented by the Secretary of the NavyApplication April 11, 1946, Serial No. 661,207

8 Claims. Cl. 318-19) This invention relates to servo systems andparticularly to long time constant servo systems.

A servo system accurately transmits mechanical information from onepoint to a remote location without mechanical connections. In somesystems, the transmitted information may fade or entirely disappear forshort intervals of time thereby causing the remote indicator indicationto jitter and fall out of synchronization with the transmitter.

It is an object of this invention to provide an electrical servo systemhaving means for smoothing the remote indicator indication, effectivelythe equivalent of a flywheel in mechanical systems.

Another object of this invention is to provide a long time constantservo system with adjustable time constant.

These and other objects will be apparent to one skilled in the art fromthe following specification when taken with the accompanying drawingwhich is generally a schematic diagram of an embodiment of theinvention.

Referring to the drawing, the output of shaft rotation data source anddetector 10, is connected through resistor 11 to the grids of amplifiertube 12 and cathode follower tube 13. The cathode of tube 12 is returnedto a positive bias source, determined by voltage division acrossresistors 14 and 15, through resistor 16. The anode of tube 12 isconnected to B+ through resistor 17 and to the grid of cathode followertube 18 whose anode is returned to +B. The cathode of tube 18 isreturned to ground through resistor 20 and to the grid of tube 12through condenser 21 and ganged switch 22A, completing a conventionalMiller feedback circuit for amplifying in effect, the size of condenser21. The anode of cathode follower 13 is connected to B+ and the cathodeis returned to ground through resistors 23 and 24 and bias controlpotentiometer 25. From the cathode of tube 13 condenser 26, gangedswitch 22B, and resistor 27 are connected in series to the movable armof potentiometer 25. Resistor 28 connects the junction of switch 22B andresistor 27 to the grid of tube 30. Converter 31, comprising asynchronous vibrator, connects and disconnects the previously describedbias source at the junction of resistors 14, 15, and 16 to the grid oftube 30 rapidly, for example, 60 times per second, thereby supplying a60 cycle carrier, modulated by the incoming signal from source 10 to thegrid. The cathode of tube 30 is returned to ground through resistor 32in parallel with potentiometer 33 and cathode resistor 34 of tube 35.The anode of tube 30 is connected to B+ through potentiometer 36.Capacitor 37 in series with resistor 38 join the anode of tube 30 andthe grid of tube whose anode is returned to B+. Resistors 40 and 41 forma voltage divider, and the junction thereof connects to the grid of tube35 through potentiometer 42, the grid of tube 46 through resistor 44,and the grid of tube 47 through resistor 45. Resistors 38 and 41together with potentiometer 42 and capacitor 43 form a low pass filterwith variable phase shift for the filter network composed of tubes 30and 35 and their associated circuits.

2,716,208" Patented Aug. 23, 1955 Capacitor couples the 60 cycleamplitude modulated sine wave to the grid to amplifier tube 46 whoseanode is connected to B through resistor 51. The cathodes of tubes 46and 47 are returned to ground through the common resistor 52. The gridof tube 47 is bypassed to ground by condenser 53 and its plate isconnected to B+ through resistor 54. The push-pull output of tubes 46and 47 is connected to the grids of push-pull output tubes 70 and 71through condenser 56 and resistor 61 and condenser and resistorrespectively. The cathodes of tubes 70 and 71 are returned to groundthrough feedback windings 65 and 66 and a common cathode resistor 64.Screen grid and plate voltage is supplied to the outpug tubes 70 and 71in conventional manner for push-pull operation. Secondary 73 of outputtransformer 72 connects to one winding 75 of two phase motor 76. Winding77 of motor 76 is connected to the same 60 cycle supply as converter 31through phase shifting condenser 80 so that the voltages applied tomotor 76 are essentially in quadrature. Direction of rotation of motor76 is determined by the phase relationships of the voltages applied toits windings. Since the phase of the voltage applied to winding 77 isfixed, it is obvious that the phase of the voltage applied to winding 75must shift 180 degrees for a reversal of motor 76. The phase of thevoltage applied to winding 75 is determined by the relative amplitude ofthe voltage applied to the grid of tube 30 from shaft rotation datasource and detector 10 and the bias voltage appearing across resistor 15of the voltage divider resistors 14 and 15. Motor 76 mechanically drivestachometer 81 which generates a voltage to balance out the appliedvoltage so that the speed of motor 76 is proportional to the appliedvoltage. Tachometer 81 is electrically connected to the grid of tube 30through resistor 82 and to the bias voltage supply appearing acrossresistor 15.

Motor 76 also drives conventional servo 83 causing motor 84 to operateand turn rotatable element 85 in synchronism with the rotated element,not shown, from which shaft rotation data source and detector 10receives rotational information. A conventional servo feedback loop isprovided from motor 84 to shaft rotation data source and detector 10.

In operation, shaft rotation data source and detector 10 suppliespositive or negative pulses of voltage when the shaft connected to thetransmitter, not shown, is not in synchronism with the remote rotatableelement 85. The amplitude and polarity of the voltage pulses dependsupon the amount rotatable element 85 is ahead or behind the transmitterrotating element, not shown.

One type of device which may be used for this purpose is a synchrocontrol transformer such as set forth at page 12-17 of Principles ofRadar by the Staff of the M. I. T. Radar School published in 1944. Whenan error exists and the transmitted information is continuous, the errorvoltage pulses are applied to the grid of tube 30 to modulate the 60cycle carrier supplied by converter 31. This voltage is filtered bytubes 30 and 35 and the associated circuit elements. Potentiometer 36determines the amplitude of the sine wave voltage fed to amplifier tube46 and inverter tube 47. The push-pull output tubes 70 and71 provide avoltage to winding 75 either leading or lagging by approximately degreesthe voltage applied to motor winding 77 for rotating motor 76. Thefeedback loop to tachometer 81 supplies sufficient negative feedback tothe modulating signal for motor 76 to run at a speed proportional to theamplitude of the input voltage ulses. p The Miller feedback circuitcomposed of tubes 12 and 18, and associated circuit elements, andcondenser 21 as described above amplifies the effective size ofcondenser 21 to produce a long time constant integrating circuit.

If the input signal from source fades or disappears for a short periodof time, this long time constant input circuit will keep the voltage atthe grid of tube at approximately constant potential thus producingsmooth rotation at rotatable element 85 instead of irregular rotation orjitter. As soon as the information from source 10 becomes steady any lagat rotatable element 85 will be almost instantly removed.

Ganged switch 22A and 22B. is adjusted to the position giving thedesired time constant, for example, A, /2 and 1 second positions.Potentiometer 25 is adjusted so that motor 76 does not operate when noinformation is being supplied by source 10. Potentiometers 33 and 42 areadjusted so that the filter output at potentiometer 36 is a sine wave.Potentiometer 36 is the gain control for adjusting the speed of motor76.

It is to be understood that the invention may be substituted for aconventional servo system as well as inserted into a working system asis indicated on the drawing.

While the specification describes what is now considered to be the mostdesirable form of the invention it will be apparent to one skilled inthe art that various changes and modifications may be made thereinwithout departing from the scope of the invention as set forth in theappended claims.

What is claimed is:

l. A long time constant servo system comprising a source of shaftrotation data, a source of alternating voltage, a long time constantinput circuit, means for combining a carrier signal and said shaftrotation data voltage. means for filtering said combined shaft rotationdata and carrier signal, means for obtaining a push-pull output voltagefrom said filtered signal, a two phase motor energized by said source ofalternating voltage and said push-pull output voltage, means forobtaining feedback from said motor to said shaft rotation data source,and a tachometer feedback circuit mechanically connected to said motorand electrically connected to the input of said filter for smoothingsaid motor rotation when said shaft rotation data information fades ordisappears for short periods of time.

2. Apparatus as in claim 1 and including a conventional servo systemconnected to said motor, a second motor connected to said conventionalservo system.

3. A long time constant servo system providing smoothing of data fromremote shaft rotation comprising, a source of shaft rotation data, asource of alternating voltage, an input from said data source connectedto a long time constant resistor and capacitor network, said capacitorconnected to a first cathode follower, a feedback circuit connected tosaid first cathode follower and said capacitor, means for changing thesize of said capacitor and said long time constant, a second cathodefollower connected to said resistor, a filter connected to said secondcathode follower, a converter connected to said source of alternatingvoltage, a tachometer electrically connected to said filter, contacts ofsaid converter shunting said tachometer, an amplifier phase converterconnected to said filter output, push-pull output amplifiers connectedto said amplifier phase inverter, a two phase motor, a first winding ofsaid motor connected to said push-pull output amplifier, second windingof said motor connected to a phase shifting condenser and to said sourceof alternating potential, a feedback loop connected between said motorand said shaft rotation data source, said motor being mechanicallyconnected to said tachometer, said motor speed being proportional tosaid input data voltage.

4. Apparatus as in claim 3 and including a conventional servo systemconnected to said motor, a second motor connected to said conventionalservo system, said second motor providing smooth operation when saidinput data fades or momentarily disappears.

5. A long time constant servo system comprising, a source of shaftrotation data signals, said shaft rotation data signals being in theform of pulses of varying polarity and magnitude, an input integratingcircuit coupled to said source of shaft rotation data signals, saidinput integrating circuit having a time constant long compared to therepetition frequency of said pulses, means for generating a relativelysquare wave of fixed frequency and of variable amplitude, the amplitudeof said square wave being a function of a signal applied to saidgenerating means, a combined filter and amplifier coupled to the outputof said square wave generating means for passing only a signal havingthe fundamental frequency of said square wave, a two phase motor havingfirst and second windings, means for supplying one winding thereof withalternating current energy having a frequency equal to said fundamentalfrequency and a phase in quadrature with said signal output from saidcombined filter and amplifier, means coupling said second winding ofsaid motor to said combined filter and amplifier, a tachometer generatormechanically coupled to said motor so as to be rotated thereby, andmeans coupling the output of said input integrating circuit and saidtachometer generator to said square wave generating means, saidtachometer generator being coupled in a direction providing negativefeedback.

6. A long time constant servo system comprising, a source of shaftrotation data signals. said shaft rotation data signals having the formof pulses of varying polarity and magnitude, an input integratingcircuit coupled to said source of shaft rotation data signals, saidinput integrating circuit having a time constant long compared to therepetition frequency of said pulses, means for generating a periodicsignal of fixed fundamental frequency, the output of said inputintegrating circuit being coupled to said periodic signal generatingmeans to modulate the amplitude of the signal generated thereby, meanscoupled to the output of said periodic signal generating means forgenerating a second periodic signal having a variable amplitude and oneof two phases, said phases differing by electrical degrees, the phaseand amplitude of said second periodic signal being a function of theamplitude modulation of said first periodic signal, a two phase motor,means for supplying one phase winding of said two phase motor withalternating current energy having a frequency equal to the frequency ofsaid second periodic signal and a phase in quadrature with said secondperiodic signal, means coupling a second phase winding of said motor tosaid means for generating said second periodic signal, a tachometergenerator mechanically coupled to said motor to be rotated thereby, theoutput of said tachometer generator being coupled back to the output ofsaid integrating means in a direction to provide negative feedback.

7. A long time constant servo system comprising a source of shaftrotation data signals, said shaft rotation data signals having the formof pulses of varying polarity and magnitude, an input integratingcircuit coupled to said source of shaft rotation data signals, saidintegrating circuit having a time constant long compared to therepetition frequency of said pulses, means for generating a periodicsignal of fixed fundamental frequency, the output of said inputintegrating circuit being coupled to said periodic signal generatingmeans to modulate the amplitude of the signal generated thereby, afilter circuit for passing only the fundamental frequency of saidmodulated periodic signal, a phase splitting amplifier circuit coupledto the output of said filter, said phase splitting amplifier beingadapted to generate second and third periodic signals at saidfundamental frequency, the relative amplitudes of said second and thirdperiodic signals being dependent upon the amplitude of the signal in theoutput of said filter, a reversible motor, means coupling saidreversible motor to said phase splitting amplifier in a manner to causesaid motor to rotate in a direction dependent upon the direction ofunbalance of said first and second signals and at a speed proportionalto the amount of said unbalance, a tachometer generator coupled to saidmotor so as to rotate therewith, the output of said tachometer generatorbeing coupled back to said means for generating said first periodicsignal in a direction to oppose the amplitude modulation of said firstperiodic signal.

8. A long time constant servo system comprising a source of shaftrotation data signals, said shaft rotation data signals having the formof pulses of varying polarity and magnitude, a capacitor type inputintegrating circuit including electronic means for amplifying theeffective capacity of the capacitor in said input integrating circuit,said input integrating circuit having a time constant long compared tothe repetition frequency of said pulses, means for generating a periodicsignal of fixed fundamental frequency, the output of said inputintegrating circuit being coupled to said periodic signal generatingmeans to modulate the amplitude of the signal generated thereby, avacuum tube filter circuit for passing only the fundamental frequency ofsaid modulated periodic signal, a phase splitting amplifier circuitcoupled to the output of said filter, said phase splitting amplifierbeing adapted to generate second and third periodic signals at saidfundamental frequency, the relative amplitudes of said second and thirdperiodic signals being dependent upon the amplitude of the signal in theoutput of said filter, a reversible motor having a difierential winding,means coupling said difierential winding of said motor to said phasesplitting amplifier in a manner to cause said motor to rotate in adirection dependent upon the direction of unbalance of said first andsecond signals and at a speed proportional to the amount of saidunbalance, a tachometer generator coupled to said motor so as to rotatetherewith, the output of said tachometer generator being coupled back tosaid means for generating said first periodic signal in a direction tooppose the amplitude modulation of said first periodic signal.

References Cited in the file of this patent UNITED STATES PATENTS2,105,598 Hubbard Jan. 18, 1938 2,300,742 Harrison et a1. Nov. 3, 19422,449,035 Coflin et a1 Sept. 7, 1948 2,470,099 Hall May 17, 1949

